1. Field of the Invention.
This invention relates to techniques for growing materials by vapor deposition, including physical vapor deposition, or PVD, and chemical vapor deposition, or CVD, techniques. In particular, this invention relates to the measurement and/or control of the composition of materials grown by vapor deposition techniques, including those used in the fabrication of integrated circuits by molecular beam epitaxy, or MBE.
2. Description of the Prior Art.
Diffraction techniques are commonly used for measuring the structure of materials, including those grown by vapor deposition. In particular, reflection high energy electron diffraction, or RHEED, techniques are commonly used for in situ measurements of the structure of a layer being grown by MBE in the fabrication of integrated circuits.
In the RHEED technique, a beam of high energy electrons, on the order of 10 keV or greater, are incident at a small angle on the surface of the layer being grown. The diffracted and reflected beams strike a target, such as a phosphor screen. The resultant pattern on the target may be used to determine the structure of the crystalline surface from which the electrons were reflected and diffracted.
Electron energy loss spectroscopy, or EELS, techniques are used for analysis of the composition of materials, such as semiconductor layers grown by MBE. EELS techniques include transmission spectroscopy in which a beam of highly collimated electrons is applied to one side of a thin layer of a material to be analyzed. The energy states of the electrons emerging from the other side of the material are analyzed. Transmission electron spectroscopy equipment has also been used in diffraction experiments in which the collimated electron beam has been applied to a target surface for composition analysis by diffraction.
Reflection electron energy loss spectroscopy, or REELS, techniques are also utilized for the analysis of material composition. The energy states of the scattered incident electrons reflected from the target surface are analyzed to determine the composition of the target. REELS techniques can be used with PVD processes and may be used with CVD processes that employ a sufficiently low operating pressure to permit passage of the incident electron beam through the gas phase in the deposition chamber.
Other surface analysis techniques include Auger spectroscopy and X-ray photoelectron spectroscopy techniques in which a beam of electrons or X-rays strikes the surface of a target layer and knocks electrons out of that target surface for analysis.
Conventional spectroscopy techniques may be used to provide valuable information about the composition of materials that have been grown by vapor deposition, but they do not provide in situ composition information during the growth of materials in deposition chambers. Such conventional techniques, unlike the above describe REELS techniques, often require short working distances, that is, the collector must be positioned extremely close to the surface of the target. This is not convenient for use with deposition techniques in which the arrangement and composition of the materials within the deposition chamber must be tightly controlled.
What is needed is a long working distance technique for the in situ analysis of the composition of materials being grown by vapor deposition. The needed technique must permit the analysis equipment to be positioned well away from the growth substrate. In addition, the technique should permit real time analysis, that is, the analysis time should be short enough so that composition information may be used in a feedback loop to control the composition of the material being grown.